1. Field of the Invention
The invention relates to a maraging steel, preferably a maraging steel without cobalt. The invention steel preferably has a high elastic limit and good ductility when aged following cold work-hardening.
2. Background of the Invention
Maraging steels are self-tempering steels which can acquire a soft martensitic structure by cooling in air, which structure can be appreciably hardened by a thermal aging treatment which gives rise to formation of intermetallic precipitates. These steels generally contain:
10-30 wt. % nickel, which enables one to obtain a martensitic structure by cooling in air; PA1 a low carbon content which enables one to obtain a soft martensite structure; and PA1 additional elements which enable hardening by formation of intermetallic precipitates, said elements being titanium, aluminum, and molybdenum, as well as cobalt, where the presence of the cobalt enhances the effects of the other added elements. PA1 Ni 17-26 wt. %, Mo 0.2-4 wt. %, Ti 1-2.5 wt. %, PA1 Al&lt;1 wt. %, and optionally some Nb, PA1 Ni 18-23 wt. %, Mo 4.5-8 wt. %, Ti 1-2 wt. %, PA1 Al 0-0.3 wt. %, C.ltoreq.0.01 wt. %, PA1 iron, and impurities; wherein PA1 the composition also preferably satisfies the following conditions: PA1 Ni 18-23 wt. %, preferably &gt;19 wt. %, and PA1 Mo 4.5-8 wt. %, preferably &gt;5 wt. %; with PA1 Ni+Mo=23-27 wt. %, preferably 24-26 wt. %, PA1 Ti 1-2 wt. %, preferably &lt;1.6 wt. %, and PA1 Al 0-0.3 wt. %,
One may also add niobium, to fix the carbon and thereby soften the un-aged martensitic structure.
Maraging steels were devised in the face of the problem of simultaneously obtaining a very high limit of elasticity and good ductility. Initially, good ductility was obtained by simultaneous addition of several wt. % of cobalt and molybdenum. However, cobalt as an alloying element is costly and not always available from a reliable source of supply. In order to avoid the constraints imposed by cobalt, maraging steels without cobalt were developed (i.e. without substantial addition of cobalt on the level set forth supra), which contain:
with the remainder being Fe and impurities resulting from the processing. Such steels are described, e.g., in Brit. Pat. 1,355,475 and U.S. Pat. No. 4,443,254; both incorporated herein by reference. These steels enable one to obtain a high tensile strength (on the order of 1800 MPa) and satisfactory ductility, in a metal which is homogenized at elevated temperature followed by cooling and aging.